The Tanoak or Tanbark-oak (Lithocarpus densiflorus) is an evergreen tree in the beech family Fagaceae, native to the western United States, in California and southwest Oregon. It can reach 40 m tall (though 15-25 m is more usual) in California's Coast Ranges, and can have a trunk diameter of 60-100 cm.
The leaves are alternate, 7-15 cm long, with toothed margins and a hard, leathery texture, and persist for 3-4 years. At first they are covered in dense orange-brown scurfy hairs on both sides, but those on the upper surface soon wear off, those on the under surface persisting longer but eventually wearing off too.
The seed is a nut 2-3 cm long and 2 cm diameter, very similar to an oak acorn, but with a very hard, woody nut shell more like a hazel nut. The nut sits in a cup during its 18 month maturation; the outside surface of the cup is rough with short spines. The nuts are produced in clusters of a few together on a single stem. The nut kernel is very bitter, and is inedible for people though squirrels will eat them.
Populations in interior California (in the northern Sierra Nevada) and the Klamath Mountains into southwest Oregon are smaller, rarely exceeding 3 m tall and often shrubby, and have smaller leaves 4-7 cm long; these are separated as Dwarf Tanoak Lithocarpus densiflorus var. echinoides. The variety intergrades with the type in northwest California and southwest Oregon.
The name Tanoak refers to its tannin-rich bark, used in the past for tanning leather before the use of modern synthetic tannins.
Tanoak is one of the species most seriously affected by Sudden Oak Death Phytophthora ramorum, with high mortality reported over much of the species' range.